Anaplasma phagocytophilum is a tick-borne obligate intracellular bacterial pathogen and the agent of granulocytic anaplasmosis in humans and animals. A. phagocytophilum primarily infects neutrophils (PMNs), subverting normal PMN apoptosis and killing mechanisms to facilitate bacterial proliferation. In contrast to endotoxin-positive bacterial infections, A. phagocytophilum infection induces neutropenia despite increased bone marrow (BM) granulopoiesis. The implication is that this granulocytotropic pathogen may fundamentally alter the upstream regulation of PMN mobilization, the downstream regulation of PMN release from BM and the systemic trafficking and homing of PMN during infection. Neutropenia and hematopoietic alterations likely contribute to the serious clinical complications of A. phagocytophilum infection, including opportunistic infections, but the mechanisms involved in altered PMN trafficking during A. phagocytophilum infection are largely unknown. Our global hypothesis is that A. phagocytophilum infection and pathogen replication perturb normal PMN trafficking and may ultimately facilitate pathogen-PMN interaction and enhance bacterial propagation. Infection-induced production of granulocyte colony stimulating factor (G-CSF) and interleukin-8 (IL-8) will be investigated as upstream regulators of PMN mobilization from BM via effects on intermediate signaling and integrin-mediated interactions. Systemic PMN trafficking and splenic homing will be defined using sophisticated imaging strategies. Mechanisms of PMN trafficking including chemotactic gradients and integrin-mediated interactions in the spleen will be determined. Our goal is to define mechanisms of infection-induced PMN trafficking and to determine their role in the innate immune response and clinical consequences of disease. The K01 award would support Dr. Jennifer Johns'career development as a postdoctoral DVM, PhD and prepare her for independent research. Dr. Johns is an experienced, board-certified veterinary clinical pathologist with a specific interest in hematologic and hematopoietic alterations in infectious disease and a commitment to advanced biomedical research. Five years of mentored support is requested.